Let me create a graph to compete with my friends--each person's data a different color line--with stakes $$ going from losers to winners.
Say your goal is to hit the gym 10 times this April.
Get three of your friends to sign up on Beeminder. Just tell them that they'll get an email a few times a week to report their gym attendance.
Then put up 20 bucks each (with your credit cards ideally...or outside of Beeminder could be OK).
And let the games begin!!! Those who stay on the yellow brick road until the end, get the spoils from the quitters.
This feature will increase adherence 100X with the added incentive of trashing your friends, and in my modest opinion, blow up Beeminder's buzz with press (4 Hour Chef readers are familiar with the stakes in DiSSS) and I would prefer Beeminder to Stickk--IF--I could bet my friends and monitor my progress on the same graph alongside my competitors.
Make it happen...before I do!!!!! : )

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Kyle Valiton commented
Glad someone has put this idea up! I'll bump this up for sure. Fitsby has got the gym down, but there are so many other great ways to utilize this idea. I for instance, would love to put up money against my business partners to knock out plans through Trello. Or for cooking meals at home vs going out. Or for making money through entrepreneurial or career goals. Thanks!
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Admindreeves (Cofounder, Beeminder) commented
Love this! And we'll get there eventually! But you should absolutely make something like this rather than wait around for us. Oh, and one of our recent additions to our competitors page -- http://blog.beeminder.com/competitors -- namely, Fitsby, might be similar to what you're envisioning.
Really appreciate you putting the idea on our feedback forum, btw. Please keep doing so!